1. Field of the Invention
The present invention relates to a flow rate control apparatus adaptable for a variety of fluids, such as gases, for use in the manufacture of such products as semiconductors, chemicals, and precision machine parts. More specifically, the present invention relates to a fluid-switchable flow rate control system that permits high-precision control of the flow rates of a variety of fluids with the use of a single orifice over specific ranges of flow rate. The orifice mounted in the present fluid-switchable flow rate control system is replaceable, so that the type of fluid and the flow rate range can be widely varied.
2. Description of the Prior Art
Mass flow rate controllers have been widely used in fluid supply systems for such manufacturing facilities as for semiconductors and chemical products.
FIG. 7 shows an example of a high-purity moisture generator for use in semiconductor manufacturing facilities. Three kinds of gases--H.sub.2 gas, O.sub.2 gas and N.sub.2 gas--are introduced into a reactor RR through valves V.sub.1 to V.sub.3, with the flow rates regulated by mass flow rate controllers MFC 1 to MFC 3. That is, first, valve V.sub.3 is opened with valves V.sub.1 and V.sub.2 kept closed, and the reactor RR is purged with N.sub.2 gas. Then, valve V.sub.3 is closed and valves V.sub.1 and V.sub.2 are opened to feed H.sub.2 gas and O.sub.2 gas to the reactor RR at specific flow rates. In the reactor RR, H.sub.2 gas and O.sub.2 gas are allowed to react into H.sub.2 O with platinum serving as catalyst without combustion. The high-purity water vapor thus produced is supplied to production facilities (not shown).
Mass flow rate controllers each have undergone linear correction and adjustment for a specific gas and flow range, and cannot be used for gases other than that for which the mass flow rate controllers are adjusted. Therefore, the mass flow rate controllers MFC1 to MFC3 are installed for H.sub.2 gas, O.sub.2 gas and N.sub.2 gas respectively, that is, one particular mass flow rate controller for one particular gas as shown in FIG. 7. Another problem is that even with the same gas, the mass flow rate controller itself will have to be replaced when the flow range, that is, the full scale flow rate is to be changed.
For a gas supply system as shown in FIG. 7, replacement is stocked for mass flow rate controllers MFC1 to MFC 3. Those mass flow rate controllers themselves are expensive and so are their replacement parts. That boosts equipment costs and running costs.
Linearity correction and adjustment takes a long time. If the linearity correction and adjustment is effected for a new gas each time types of gases and flow ranges are changed instead of replacing the mass flow rate controller, the production plant has to be temporarily shut down. For this reason, it is necessary to have spare mass flow rate controllers in stock at all times.